1. Field of the Invention
The present invention relates to communications equipment and, more specifically, to a signal receiver, such as a quadrature modulation receiver.
2. Description of the Related Art
The use of quadrature modulation is a common communication technique for transmitting data, such as digitized audio streams. More specifically, the data to be transmitted are encoded using (1) an “in-phase” (I) modulation signal and (2) a “quadrature-phase” (Q) modulation signal that has a 90-degree phase difference with the in-phase modulation signal. The I and Q modulation signals are combined to form a composite I/Q signal, which is then transmitted over the communication channel.
One example of a quadrature modulation communications system is a system that complies with the IEEE 802.11a/g standards. More specifically, the IEEE 802.11a/g standards specify an orthogonal frequency division multiplexing (OFDM) scheme that employs a combination of frequency division multiplexing and quadrature modulation. At an OFDM transmitter, a plurality of composite I/Q signals (OFDM sub-carriers) are frequency-division-multiplexed for transmission to a corresponding OFDM receiver so that, in the frequency domain, the primary peak of each OFDM sub-carrier coincides with a null of every other OFDM sub-carrier. As a result, in the transmitted signal, not only are the I and Q components of each OFDM sub-carrier orthogonal to one another, but also different OFDM sub-carriers are mutually orthogonal as well. These multiple orthogonality relationships advantageously enable an OFDM communications system to reliably demodulate the transmitted signal at the receiver, while operating at relatively high spectral data-transmission density.
One problem with quadrature modulation communications systems in general and OFDM systems in particular is that they are relatively sensitive to deviations from the above-specified orthogonality relationships. As a result, a quadrature modulation transmitter/receiver usually incorporates one or more calibration sources that are used to maintain signal orthogonality at various modulation/demodulation stages. However, disadvantageously, these calibration sources can take up a significant die area, thereby increasing the size and/or cost of the chip set implementing the transmitter/receiver.